The present invention relates to a high-density semiconductor memory device having a stacked capacitor structure and a method of manufacturing the same, and more particularly, to a data storage capacitor having an improved charge storage electrode and a method of manufacturing the same.
In the field of DRAMs (Dynamic RAMs) having capacitors for holding data, various kinds of techniques have been developed to ensure a given data storage charge amount to deal with the decrease of the cell area resulting from miniaturization. An example is disclosed in H. Watanabe et al., "An Advanced Fabrication Technology of Hemispherical Grained (HSG) Poly-Si for High Capacitance Storage Electrodes", Extended Abstracts of the 1991 International Conference on SSDM, pp. 478-480, 1991. This reference describes a so-called roughened surface technology (to be referred to as an "HSG technology" hereinafter) with which a polysilicon film as an electrode of a capacitor is annealed in vacuum to form small hemispherical grains on the surface of the polysilicon film. An example in which this HSG technology is applied to a DRAM is also known. This example is disclosed in, e.g., "A Capacitor-Over-Bitline (COB) Cell with A Hemispherical Grain Storage Node for 64 Mb DRAMs", IEDM90 Technical Digest, pp. 665-658, 1990, or "Method of Forming A Capacitor", U.S. Pat. No. 5,444,013.
With the HSG technology, the surface area of a polysilicon film as an electrode of a capacitor can be increased. As a consequence, the capacitance required to store data increases, and a desired storage charge amount can be ensured.
When a silicon nitride film is used as the dielectric film of a capacitor, polysilicon is normally used as an electrode material. The HSG technology is advantageous in increasing the electrode area of a capacitor when polysilicon is used as an electrode material.
A method of forming a rough or undulated surface on a polysilicon film as the lower electrode (storage electrode) of a capacitor using the HSG technology will be briefly described. As shown in FIG. 39, an insulating interlayer 2 is formed on a semiconductor substrate (not shown), and then, a contact plug 3 connected to an element region on the semiconductor substrate is formed. Polysilicon is deposited at 550.degree. C. by, e.g., low-pressure CVD. The deposited polysilicon is patterned by the conventional lithography and RIE to form a polysilicon film 601 as the lower electrode of a capacitor. Subsequently, as shown in FIG. 40, after a natural oxide film on the polysilicon film is removed by a diluted HF solution, the resultant structure is annealed in vacuum to obtain a polysilicon film 602 with a rough or undulated surface. With this process, a capacitor having large electrode area and capacitance can be obtained.
Another technique of improving the capacitance to ensure a given data storage charge amount is known. For example, "Giga-bit Scale DRAM Cell with New Simple Ru/(Ba,Sr)TiO.sub.3 /Ru Stacked Capacitors Using X-ray Lithography", IEDM95 Technical Digest, pp. 903-906, 1995 discloses a technique using a high-dielectric film of (Ba,Sr)TiO.sub.3 (to be referred to as a "BSTO" hereinafter) having a high dielectric constant as the dielectric film of a capacitor.
As a semiconductor device becomes further minute in size in feature, both the electrode area of a capacitor and the dielectric constant of a dielectric film need be improved. A technique of combining an electrode having a rough or undulated surface and a high-dielectric film to form a capacitor meets this requirement.
A high-dielectric film such as a BSTO film is a metal oxide film. In the process of depositing a metal oxide film, active oxygen is contained in the atmosphere. When polysilicon is used for the electrode of a capacitor, and BSTO is used for the dielectric film, the polysilicon surface is oxidized during deposition of BSTO to form an SiO.sub.2 film having a low dielectric constant. As a result, the capacitance becomes low. Hence, in the prior art, when a metal oxide such as BSTO is used for the dielectric film of a capacitor, a metal such as platinum (Pt) or ruthenium (Ru) must be used as an electrode material.
However, in the use of the above-described HSG technology, a rough pattern can be formed on a polysilicon surface, but it is not possible to form rough pattern on a metal surface. For this reason, when a metal such as platinum or ruthenium is used as the electrode material of a capacitor, the electrode area can hardly be increased.